Various denitration techniques designed to reduce and transform NOx in flue gas into harmless N2 by supplying ammonia gas have heretofore been proposed. It was known that an application temperature of these techniques generally falls in a range from 800 to 1000° C. If the ammonia gas is supplied to the flue gas at 1000° C. or above, the NOx is rather increased as a result of combustion of the ammonia gas. On the other hand, if the ammonia gas is supplied to the flue gas at 800° C. or below, the denitration does not progress properly due to a slow reduction reaction rate.
In order to carry out denitration, it is preferable to set an installation location of a nozzle for supplying the ammonia gas into the flue gas in a region where the flue gas having a temperature range from 800 to 1000° C. flows. Nonetheless, there is usually no room in this region because a group of heat exchangers is supposed to be installed thereat. It is therefore difficult to install the nozzle for supplying the ammonia gas in this region, and the nozzle would typically have to be installed in a space in another region where flue gas having a temperature of 1000° C. or above flows immediately after combustion.